In one known kind of printer, adapted to print on a print medium as it passes over a platen using a plurality of print elements operated selectively by hammers, the print elements are mounted on flexible fingers forming part of a metal band in the form of a continuous loop, one print element being mounted on each flexible finger. The print elements extend in a straight line along the band parallel to the longitudinal center line of the band. A bank of hammers extends along the platen and is spaced from the platen so as to define a print region between the hammer bank and the platen. The print medium extends through the print region across the platen so that the hammer bank extends across the width of the print medium. The metal band on which the print elements are mounted also extends through the print region along the platen and across the width of the print medium and is located between the hammer bank and the print medium. An ink ribbon also is located in the print region between the metal band and the print medium.
The metal band is driven continuously along the platen past the hammer bank and across the print medium by a suitable drive system. Operation of any one of the hammers of the bank causes the hammer to move towards the metal band and to abut against one of the print elements so as to move this print element on its flexible finger towards the ink ribbon and the print medium. The print element in moving abuts against the ink ribbon and forces the ink ribbon into contact with the print medium causing the printing of a mark on the print medium of the shape of the print element.
According to one particular printer of the above kind and illustrated by way of example in U.S. Pat. No. 4,428,284, each of the print elements is shaped like a dot and operation of each hammer causes the printing of a dot on the print medium. As the metal band moves continuously across the print medium operation of selected hammers will result in the printing of a line of dots in positions on the print medium corresponding to the positions of the hammers which are operated. Each hammer is formed with a head which has a width in the direction of movement of the band which is greater then the width of a single dot. It is therefore possible for each hammer to print a dot in any position on the print medium which is covered by the hammer by varying the timing of the operation of the hammer relative to the movement of the band. As a result each hammer can print a dot in a number of positions on the print medium. Therefore, the dots in the line printed can occupy many selected positions on the print medium. There is only a small gap between each pair of adjacent hammers and the hammers can print dots at all required positions along the line printed.
After one line of dots has been printed the print medium can be moved through a small increment transversely to the length of the platen and the operation can be repeated resulting in the printing of a second line of dots below the first line of dots. By repeating these operations lines of dots can be printed as required.
A character can be printed on the medium by printing dots in selected positions in a matrix, for example a matrix of 5 columns and 7 rows. By printing dots in selected positions in the lines as described above, characters can be printed in selected positions on the medium. In order for the characters to be printed correctly it is essential that the positions of the dots should be defined accurately. For this to happen it is essential that the metal band should move accurately relative to the platen and the bank of hammers so that the positions of the print elements can be accurately determined. It is therefore necessary to provide a suitable means for driving the band along the platen through the print region and to control the band as it is driven.
It is known to drive the metal band by passing the band around the periphery of a single drive wheel located at one end of the hammer bank and to rotate the drive wheel by means of a suitable motor and thereby draw the band along the hammer bank towards the drive wheel. The band as it moves past the hammer bank will abut against and be guided by the hammer bank. Since the band is continuous in the form of a loop and has an inherent stiffness it will extend along the hammer bank from the end of the hammer bank at which the drive wheel is located to the other end of the hammer bank and will curve back on itself so that it extends back to the drive wheel without any further guiding means at the other end of the hammer bank. The band can be pressed against the surface of the drive wheel by means of a roller on the end of a pivoting arm which is biased towards the drive wheel. Such an arrangement is described in the above mentioned United States Patent.
It is essential that the band moves evenly along the platen past the hammer bank. The contact between the band and the periphery of the drive wheel must be good so that the driving force provided by the rotation of the drive wheel is evenly imparted to the band. However the frictional force exerted on the band by the roller pressing the band against the periphery of the drive wheel is not always satisfactory for this purpose. It is known to provide a drive wheel of the above type with a permanent magnet which attracts the band as it passes around the periphery of the wheel as an alternative to using the roller pressing the band against the periphery of the wheel.
As an alternative to using only a single drive wheel for the band it is possible to provide in addition a freely rotating idler wheel at the end of the hammer bank remote from the drive wheel and to curve the band around the periphery of this idler wheel. By suitably positioning this idler wheel a tension can be exerted in the band. However it is essential that the axes of the drive wheel and the idler wheel should be accurately aligned in order to ensure that the band moves accurately and evenly past the hammer bank and to prevent stresses being exerted in the band and this alignment is not always easy to achieve.
It is also essential that the band be prevented from flexing laterally so that it remains flat as it moves along the platen past the hammer bank. Without any additional guiding means located at the end of the hammer bank remote from the drive wheel there is no tension on the band and as a result the band tends to flex as it moves. It is known to guide the band as it passes along the hammer bank by suitable guide rollers located at the lower edge of the band and against which the band is pressed by means of a suitable arm forcing the band downwards towards the rollers. This reduces the tendency of the band to flex laterally. However with such an arrangement if the force is great enough to ensure good contact between the band and the rollers in order to reduce the tendency to flex laterally there is a possibility that the band will buckle. This will result in uneven movement of the band.
It is also known to use permanent magnets to control other kinds of print element holders as they are moved relative to hammers, for example those kinds in which the print elements are connected together to form a chain. By way of example such an arrangement is described in U.S. Pat. No. 3,435,756.
The object of the present invention is to provide a printer of the type including a bank of hammers and a metal band on which are mounted print elements having improved means for controlling the movement of the band through the print region of the printer past the hammer bank.
A further object of the invention is to ensure that the movement of the band is controlled in such a way that the print elements are aligned with the hammers in the hammer bank over the whole length of the hammer bank.